Building docker from source for Raspberry Pi

Not totally bootstrapped yet, since this depends on a running copy of docker v0.6.4, which can in turn be installed via the [script from Resin.io](http://resin.io/docker-on-raspberry-pi-in-4-simple-steps/)

What this adds to that script is the ability to build more recent versions of Docker (assuming these changes have been merged/rebased on top)